a. Field of the Invention
This invention relates to an electrophotographic process, and, more particularly, it is concerned with the electrophotographic process for forming an image by use of a photosensitive plate having a plurality of openings.
b. Discussion of Prior Arts
As the typical conventional electrophotography, there have been proposed a direct process such as, for example, electrofax, and an indirect process such as xerography. In the direct electrophotographic process, use is made of a specially treated image recording member coated with a photoconductive material such as zinc oxide. This direct method, however, has a drawback in that when the image formed on the recording member locking brightness, contrasts in the tones of the reproduced image are poor. Moreover, owing to a particular treatment rendered on the recording member, it is heavier than the conventional paper, hence a particular feeding means which is different from that for ordinary paper should be employed. According to the indirect process, an image of high contrast and good quality can be obtained by using ordinary paper as the image recording member. However, in this indirect process, when a toner image is transferred to the recording member, the latter inevitably contacts the surface of the photosensitive member, and, further, cleaning means vigorously touches the surface of the photosensitive member for removal of the residual toner thereon with the consequence that the photosensitive member is impaired every time the transfer and cleaning operations are carried out. As the result of this, life of the expensive photosensitive member becomes shortened, which unavoidably ensues a high cost in the image reproduction.
In order therefore to remove such drawbacks inherent in the conventional electrophotographic processes, there have been contemplated various methods such as, for example, those taught in the U.S. Pat. Nos. 3,220,324, 3,645,614, 3,647,291, 3,680,954, and 3,713,734. In these patents, there is used a photosensitive member of the screen type or grid type having a number of openings in the form of fine net. The electrostatic latent image is formed on the recording member by modulating flow path of ions through the screen or grid, after which the latent image formed on the recording material is visualized. In this case, the screen or grid which corresponds to the photosensitive member need be either developed or cleaned, hence the life of the screen or grid can be prolonged.
U.S. Pat. No. 3,220,324 teaches use of a conductive screen coated with a photoconductive material, through which an image exposure is effected onto the recording member simultaneously with the corona discharge. The flow of corona ions produced as the consequence of the corona discharge is modulated by the screen, whereby an electrostatic latent image is formed on the recording member. In this process, wherein the screen charging and the image exposure are simultaneously effected, it is difficult to charge the photoconductive material coated on the conductive screen at a sufficiently high potential. Accordingly, the efficiency in the image exposure becomes lowered to make it difficult to obtain the image reproduction at a high quality. Further, at the dark image portion where the corona ions pass, if the potential to the conductive screen is raised too high, the applied corona ions are repulsed with the consequence that they are directed to the bright image portion in the vicinity of the dark image portion of the exposed conductive screen, hence no satisfactory image reproduction can be expected.
U.S. Pat. No. 3,680,954 teaches use of a conductive grid coated with a photoconductive material, and a conductive control grid, in which an electrostatic latent image is formed on the conductive grid, and different electric fields are formed on both conductive grid and control grid so as to modulate flow of the corona ions for forming an image on the recording member. In this patented process, however, it is quite difficult to hold the control grid and the conductive grid to form an electrostatic latent image over a large area with fine space intervals therebetween. Moreover, the control grid absorbs the corona ions to be imparted to the recording member with the result that the image recording efficiency becomes lowered. In the case of forming a positive image, the flow of the corona ions having a polarity opposite to that of the latent image is applied, and almost the entire part of the ion flow directs to the latent image to negate the latent image, so that the desired positive image is difficult to be reproduced.
In U.S. Pat. No. 3,645,614, the screen comprises an insulating material overlaid with a conductive material, and the insulating material comprises a photoconductive material. An electric field to prevent the ion flow from passing through the screen is formed at the openings or perforations for permitting the ion flow to pass therethrough owing to the electrostatic latent image formed on the screen. This process has a drawback in that an image to be formed on the recording member is the image reversal of the latent image on the screen.
U.S. Pat. No. 3,713,734 teaches use of a four-layer screen consisting of a photoconductive substance, a first conductive substance, an insulating substance, and a second conductive substance, in which an electrostatic latent image is formed on the photoconductive substance in conformity to the original picture image by the processes of electric charging and image exposure. Also, in the case of forming an image on the recording member by modulating the flow of the corona ions through the electrostatic latent image, the second conductive substance of the screen is imparted by a voltage having a polarity opposite to that of the electrostatic latent image on the screen, since the image is in a single polarity. By this application of the electric field, there are formed two regions, i.e., a region to permit the ion flow to pass through the screen in accordance with the latent image on the screen, and another region to inhibit the passage of the ion flow, whereby a desired electrostatic latent image is formed on the recording member. According to this patented process, it is possible to reproduce a favorable positive image, although the process has two major disadvantages such that two layers of the conductive substance must be provided on the thinly formed screen, which entails complexity in the manufacture of such screen, and that instability remains between the facing layers of the conductive substance owing to electric discharge. Further, the electric charge on the photoconductive substance layer is liable to attenuate, and the configuration of the layer tends to largely fluctuate in the course of its manufacturing, on account of which it becomes difficult to obtain a persistent electrostatic latent image on the photoconductive substance layer over a long period of time, and to modulate the ion flow for many repeated times by the electrostatic latent image on the one and same screen.
U.S. Pat. No. 3,647,291 teaches the formation of electrostatic latent images having mutually different polarities on a two-layer screen consisting of a conductive substance and a photoconductive substance in correspondence to a bright image portion and a dark image portion so as to modulate passage of the corona ion flow by the latent image formed on the screen. However, with this patented method, as described in its specification, it is very difficult to form a latent image of both polarities on the photoconductive insulating substance in laminar form. Rather, in the case of forming the electrostatic latent image on this laminar insulating substance, it is necessary to transfer the latent image once formed on a separate photosensitive body. That is, according to the patented method as outlined above, there takes place an electric charge loss in the course of the image forming process, and the construction of the electrophotographing device becomes inevitably complicated. In particular, in case the electrostatic latent image is to be transferred onto the screen from the photosensitive body, the latent image tends to flow toward the conductive substance which has been exposed at the side of the screen openings, on account of which the desired electrostatic latent image can hardly be obtained on the screen with satisfactory contrast in the tones of the image.